1. Heat olive oil over medium heat in a large frying pan. Add onion and 1/2 teaspoon salt, and sauté for five to eight minutes, until lightly browned.
2. Add garlic and butter and sauté for an additional two to three minutes. Add spinach and cook until the liquid cooks off, about five to eight minutes.
3. Add heavy cream and season with remaining one teaspoon salt and pepper. Stir in feta and roasted tomatoes.
4. Divide the stuffing among the portobello mushroom caps and top with the seasoned panko mixture. Return to oven and roast for 15 minutes.